Yes sir, you are correct. Thank you for correcting me. In practice, a
NULL in a foreign key almost always means an orphan record. In most
applications the designer will want to use the NOT NULL constraint.
Bob Hall
>A foreign key value can be null, if it suits the data application - or that
>is what I've always been taught.
>
>Here is a text book definition of Referential Integrity which was spoon fed
>to me by the Open University when studying their RDBMS course:
>
>"Referential Integrity - If a relation (table), R2 has a foreign key, F,
>that references the primary key, P, in another relation (table), R1, then
>every R2.F entry must either be a value equal to an R1.P primary key value
>or be null"
>
>Regards,
>Chris
>
> > -----Original Message-----
> > From: Bob Hall [mailto:[EMAIL PROTECTED]]
> > Sent: 31 January 2001 01:53
> > To: [EMAIL PROTECTED]
> > Subject: Re: [PHP-DB] foreign key problem
> >
> >
> > >hello all!
> > >i have a little problem ,hope someone can help me out
> > >the problem is :
> > >let i have two tables T1 and T2 ,now T1 has following
> > >fields
> > >cntryid
> > >cntryname
> > >cntrycode etc.
> > >now cntryid is the primary key now i want to make the
> > >cntryid a foreign key in table T2 .so how can i do it
> > >plz help me .
> > >msjamal
> >
> > A column is a foreign key because it contains only values found in
> > the referenced key, and no NULLs. Design your database so that the
> > foreign key column contains only values from cntryid.
> >
> > If you want to know how to declare a referenced key/foreign key
> > relationship, you will have to specify what RDBMS you are using.
> >
> > Bob Hall
> >
> > Know thyself? Absurd direction!
> > Bubbles bear no introspection. -Khushhal Khan Khatak
> >
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>
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